Several trends are apparent in modern wireless communication systems. Firstly, User Equipment (UE) capabilities and processing power have increased considerably. This is driven, in part, by the development of the long-term evolution (LTE) advances to wireless communication protocols, such as Global system for mobile communications (GSM) and Universal Mobile Telecommunications System (UMTS), in order to support high peak data rates. The trend is also driven by the multi-carrier (MC) evolution within Wideband CDMA (Code Division Multiple Access)/High Speed Packet Access (WCDMA/HSPA).
Secondly, main-remote network architectures, in which multiple cells located at different physical locations share a baseband unit (and which enables coordination between the cells), are becoming increasingly popular.
Thirdly, the user demand for high peak data rates, and operators' desire to manage their wireless resources efficiently, have both increased and continue to do so. This is a consequence of mobile operators having begun to rely on WCDMA/HSPA technology to offer mobile broadband services.
Observation of these trends has triggered discussions in the Third Generation Partnership Project (3GPP) on standardizing support for multi-cell transmissions. Several multi-cell transmissions techniques have been discussed in 2010, including switched transmit diversity techniques, multi-flow transmission techniques, and single-frequency network transmissions.
Data-discontinuous transmission techniques include HS-DDTx (High Speed Data Downlink Transmitter), as described in R1-104913, “Multi-cell transmission techniques for HSDPA (High-Speed Downlink Packet Access), Nokia Siemens Networks, Nokia, and SF-DC-HSDPA (Single Frequency Dual Cell HSDPA) switching, as described in R1-104738, “Further details and benefits of deploying DC-HSDPA (Dual Cell HSDPA) UEs in Single Frequency Networks”, Qualcomm Europe. This class of techniques is based on HSDPA transmissions from different cells being coordinated so that the inter-cell interference is reduced.
Multi-flow transmission techniques include SF-DC-HSDPA aggregation. This class of techniques is based on several, independent data streams being transmitted to the same UE from different cells located at the same or different sites.
Single-frequency network transmissions include HS-SFN (High Speed Single Frequency Network). This technique is based on identical data to the same UE being transmitted from multiple cells simultaneously. The transmitted data is combined in the “air,” and the UE consequently benefits from a stronger received signal.
Downlink multi-point transmission techniques are mainly useful when the signal strengths for multiple cells are comparable and the non-serving cells are partially loaded. There are two modes in the SF-DC-HSDPA: SF-DC Aggregation and SF-DC Switching.
In SF-DC Aggregation, either of dual cells can simultaneously transmit different transport blocks to the same UE; the two cells can belong to the same NodeB (Intra-NodeB aggregation) or different NodeBs (Inter-NodeB aggregation); and The SF-DC UE with advanced receiver can gain from this approach by suppressing interference between flows.
In SF-DC Switching, only one of dual cells can transmit a data block to the UE at a given TTI (Transmission Time Interval); the better cell (e.g. a cell with higher CQI (Channel Quality Indicator)) is selected to transmit data; the H-ARQ (Hybrid Automatic Repeat Request) retransmission to a UE can be scheduled in either cell; and the UE with a less advanced receiver can also gain from this approach.
For both of these cases, the HS (High Speed) timing between the two cells may be asynchronous; the UE monitors HS-SCCH (High Speed Shared Control Channel) from both cells; and the ACK/NACK (Acknowledgement/Negative Acknowledgement) and CQI information for either cell are reported jointly.
To determine the downlink multi-point transmission, the network can reuse existing events (e.g., Event 1a/b) and measurements (e.g. scheduling information (SI) or CQI). Alternatively new measurements and events could be defined. At RAN (Radio Access Network) #50 a study item on downlink multi-point transmission techniques was approved. See RP-101439, “Proposed study item on HSDPA multipoint transmission”.
The communication performance can be different for these different modes in different situations. How to determine the optimal mode for a given UE, and how to configure the UE for the selected mode, is thus a challenge.